Contractile force in muscle fibers is generated by crossbridges that form between the myosin-containing and actin-containing filaments and hydrolyze ATP. During shortening, the two types of filaments slide relative to each other. Using skinned fiber preparations from rabbit, structural changes associated with crossbridge formation and strain were studied by X-ray diffraction, electron microscopy, and mechanical methods. A possible mechanism for force development in rigor muscle fibers subjected to lateral electrostatic expansion was discovered by electron microscopy. The thick, myosin- containing filaments were found to separate into three subfilaments in the periphery of the A-band when lattice expansions occurs. The associated decrease in axial length of the subfilaments could account for the significant axial forces that appear under these conditions. Cryomicroscopy and image processing were used to measure the mass associated with the thick and then myofilaments in relaxed and rigor rabbit muscle fibers. Mass transfer from the thick to the thin filament took place when relaxed fibers passed into rigor. Phases were calculated for the first five equatorial X-ray reflections in the relaxed and rigor states. Gel electrophoresis showed that muscle fibers from Duchenne muscular dystrophy patients contain normal amounts of all the muscle proteins (including titin and nebulin). Absence of nebulin does not appear to be the cause of this disease. Radiation inactivation analysis of force development by muscle fibers at various sarcomere lengths suggests that titin and nebulin may directly affect the force output of actomyosin crossbridges as well as order of the thick filaments in activated muscle fibers. The overall goal of the project is to find out how the actomyosin system in muscle fibers produces force and motion, and how the mechanical responses are regulated.